The present invention relates to a cold trap for removing impurities from liquid sodium which is circulated, as a heat exchanging medium, in atomic energy plant having a fast breeder.
In atomic energy plant having a fast breeder, the liquid sodium used as the heat exchanging medium is circulated in a closed loop including the reactor core of the fast breeder and an intermediate heat exchanger or a steam generator, through a piping having various valves and pumps, so as to receive the heat from the reactor core and deliver the same to water, thereby to generate steam. Thus, the circulation of the liquid sodium is continued without any suspension, as long as the breeder is working.
During the circulation of the liquid sodium, the concentration of impurities is gradually increased due to generation of sodium oxides by oxygen contained by the liquid sodium and accumulation of metallic impurities, fission products and the like. These impurities are carried and conveyed by the liquid sodium, as the latter is circulated, incurring various troubles in the associated parts of the sodium circulation system. For instance, the impurities attaching to the external surface of fuel pin in the reactor core may cause a local overheating of the fuel pin, resulting in a breakage of the latter. Also, the scale of impurities attaching to the surface sodium handling apparatus is liable to promote the corrosion of the portion of the apparatus to which the scale attaches.
As is well known, these impurities are precipitated and separated as the temperature of the liquid sodium is lowered. In order to avoid above-stated troubles, through removing the impurities by making use of this phenomenon, it has been proposed to incorporate in the sodium circulation system a cold trap adapted to cool the liquid sodium and to separate the precipitated impurities from the cooled liquid sodium, so as to continuously purify the latter.
The cold trap usually has a vessel having a sodium inlet and outlet and a passage of sodium disposed in the vessel. The sodium is cooled as it flows along the passage, and the precipitated impurities are trapped before the liquid sodium is returned to the sodium circulation system.
To explain in more detail about a typical conventional cold trap, by way of example, the liquid sodium to be purified, which has been introduced into the vessel through the sodium inlet port formed at an upper portion of the vessel, is made to flow downwardly along an annular passage formed by the inner peripheral surface of the vessel. As the liquid sodium flows down along the annular passage, it is kept in contact with cooling means disposed in the annular passage, so as to be cooled by a cooling gas which is circulated in the cooling means. Among the impurities precipitated as a result of the cooling, the solid impurities of relatively large particle sizes are deposited to and accumulated on the bottom of the vessel, while the impurities of relatively small particle sizes are made to flow together with the liquid sodium. The liquid sodium then makes a turn at the bottom of the vessel and then flows upwardly through a central passage formed in the center of the vessel. In the central passage for the liquid sodium, disposed is a wire mesh made of stainless steel, so as to trap the aforementioned impurities of small particle sizes. The purified liquid sodium then leaves the wire mesh and flows back into the sodium circulation system, through the sodium outlet formed in the center of the vessel. This type of cold trap is disclosed in Japanese Patent Publication No. 32051/1975.
This type of cold trap, however, involves various problems. Namely, since the impurity-trapping means is disposed in the central passage surrounded by the annular passage, the cooled liquid sodium is inconveniently reheated as it passes the upper part of the impurity-trapping means, so as to dissolve again not a small part of impurities which have once precipitated at a cost of cooling labour. The impurities dissolved again in the liquid sodium can never be trapped by the impurity-trapping means, and are conveyed again into the sodium circulating system, together with the outlet flow of the liquid sodium. This reheating of the liquid sodium is caused by the heat which is transferred to the liquid sodium in the impurity-trapping means, from the liquid sodium in the area around the sodium inlet at which the sodium temperature is still considerably high.
It has therefore been proposed to form a heat insulating layer filled with a gas, between the outer annular sodium passage and the impurity-trapping means. This however requires a complicated construction of the cold trap, and the cost of production of the cold trap is uneconomically raised for ensuring a sufficiently high reliability. In addition, if the structure for forming the heat-insulating gas layer is produced as a pressure-resistant vessel, the wall thickness of the structure is increased impractically. To avoid this, it is necessary to adopt such a structure as to allow a communication between the heat-insulating gas layer and the cover gas in the trap vessel, so as to establish a balance of pressure therebetween. Usually, the filling of the sodium circulating system with the liquid sodium after the building of the plant or after a periodical survey of the same is made conveniently by at first evacuating the sodium circulating system and then sucking the liquid sodium into the system by means of the vacuum. It is impossible to enjoy such a convenience in filling the sodium circulating system, when the above-mentioned communication between the heat-insulating gas layer and the cover gas in the trap vessel is allowed.
In addition, it is necessary to reheat the cooled sodium before it is returned to the sodium circulating system, for otherwise the cold liquid sodium may impart a thermal impact to the hot piping of the sodium circulation system. Conventionally, this reheating of the liquid sodium has been made by a specific economizer installed separately.
Under these circumstances, there is an increasing demand for a cold trap having a simple construction and, at the same time, capable of trapping the whole part of once-precipitated impurities without fail and recovering the high temperature of the liquid sodium before it is returned to the sodium circulation system.